1. Field of the Invention
The invention relates to a proton conductive membrane and a production method thereof.
2. Description of the Related Art
As electrolyte material for a proton-exchange membrane fuel cell, polymer containing perfluorocarbon sulfonic acid such as Nafion (trademark) is widely employed. The electrolyte that constitutes a membrane-electrode assembly (hereinafter, referred to as an MEA) of the fuel cell needs to have proton conductivity and an ability to prevent crossover of reaction gases at both electrodes, that is, a gas-barrier property.
The polymer containing perfluorocarbon sulfonic acid such as Nafion has high gas permeability. Therefore, when the polymer containing perfluorocarbon sulfonic acid is used as the electrolyte membrane, there arise problems concerning a decrease in performance of the electrolyte membrane and durability of the electrolyte membrane. Further, there arises another problem that a distance between polymer chains of the polymer electrolyte is increased due to wetting during electric power generation, and the gas-barrier property deteriorates.
Accordingly, the inventor has focused attention on a thin film using clay mineral, as material having a high gas-barrier property. A layered clay mineral thin membrane is disclosed, which is produced by preparing a spreading solution containing layered clay mineral including an aluminum skeleton and a compound containing a phosphate group, spreading the spreading solution on a substrate, and then removing a solvent from a liquid membrane on the substrate (Japanese Patent Laid-Open Publication No. 5-254824, and M. Isayama Chem. Lett., 1283 (1993)).
However, the layered clay mineral thin membrane produced by the aforementioned production method has a low ion-exchange ability (that is, low proton conductivity). Therefore, the layered clay mineral thin membrane is not satisfactory material that can be used as the electrolyte membrane of the fuel cell.